In order to measure the biosynthesis of a macromolecule or biopolymer composed of subunits, it is necessary to know the degree of labeling (specific activity or enrichment) of its biosynthetic precursor units. In simplest terms, the turnover rate of a biosynthetic product is determined by the rate at which its specific activity or enrichment approaches the specific activity or enrichment of its precursor. Mathmatically, this represents the asymptotic value toward which the product approaches (Waterlow).
Similarly, the fraction of a product derived from a precursor is calculated from the ratio of their specific activities or enrichments (Wolfe). The experimental inaccessibility of intracellular precursors coupled with the complexity of subcellular biochemical organization and the existence of discrete pools of precursors complicates investigation of the turnover of macromolecules, including proteins, lipids, nucleic acids and carbohydrates.
The essence of the problem is that the functional ("true") precursor for a macromolecule may come from a special subcellular pool which may not be readily isolated using biochemical fractionation techniques. An experimental solution to this problem would be to measure the true precursor specific activity or enrichment for each product molecule without having to isolate biochemically the actual intracellular precursor molecule involved. True endogenous synthesis of the product could then be determined.